The present invention relates to ribbons for non-impact printing and more particularly to ribbons for non-impact printing of magnetic ink compositions to encode checks and other machine readable documents.
So called "non-impact printing" as a broad concept is now well known in the art. It has become a more and more popular means of printing in typewriters, computer printers and the like because of the elimination of the very high noise associated with impact technologies such as dot matrix and daisy wheel systems. The basic principle of the non-impact printing is the use of heat to melt an ink coating from the ribbon to form an image on a receiver substrate such as paper.
The conventional thermal transfer process employs a thermal printhead which is a resistor, and the ribbon is composed of a substrate of polyester film with a wax ink coating applied to one side. The printhead generates a thermal energy which comes in contact with the polyester. The heat is transmitted from the printhead through the polyester to the wax ink coating which melts to form the image. The thermal printhead must, of course, be cooled down and reheated for each separate image formation.
A more recent non-impact system, often referred to as an electrically resistive heat transfer system differs from the conventional thermal transfer system both in printhead and in ribbon construction. Using this technology, the printhead is not a resistor and does not itself generate heat per se, but rather is composed of a plurality of thin wires or electrodes which pass on electrical current. The heat needed for production of the image is generated within the ribbon itself by the electrical current from the printhead. Thus, the ribbon itself is in effect the resistor and normally comprises three layers, a conductive polymer film which will serve as a resistor with respect to the electric current and thereby generate heat; a thin layer of metal such as aluminum usually applied by vacuum deposition techniques; and the third ink containing meltable polymer based layer which will melt in response to the heat generated in the polymer film, and transfer from the metal layer to the substrate in the form of the desired image. An additional release layer is sometimes employed between the aluminum and the ink layer to further facilitate the transfer of the ink to the substrate.
The electrically resistive heat transfer techniques have a number of significant advantages over so-called conventional thermal transfer techniques. First, they substantially lower the printer costs, since they eliminate the necessity for expensive components to cool and reheat the printhead. Also, they facilitate higher printing speeds since they don't require a conventional resistor thermal printhead which must be cooled down and reheated between images. And, perhaps most important, these new techniques can generate better print quality, since the heat is generated within the ribbon itself and is not dissipated by going through intermediate layers, thereby providing better print quality over a much wider range of papers, films and other substrates.
To date, however, the materials employed in the ink layer of electrically resistive heat transfer ribbons have consisted primarily of pigments such as carbon black and other inorganic materials.
For example, U.S. Pat. No. 4,103,066 discloses a ribbon for non-impact printing which comprises a transfer layer and a substrate. The substrate is a polycarbonate resin containing from about 15 to about 40% electrically conductive carbon black and the transfer coating is made up of wax, carbon black and a dye such as methyl violet dye. U.S. Pat. No. 4,549,824 discloses the use of azo dyes in thermal ink transfer applications, but these dyes facilitate the use of lower temperatures rather than providing erasure proof print characters on the ultimate substrate.
While the inks and ribbons heretofore known are quite satisfactory in typical conventional typing and printing applications of most business offices, they are often unsuited for applications such as the printing of checks, negotiable instruments and other special documents of the type which should, if possible, be erasure proof and which can only be expeditiously handled by sophisticated magnetic reader/sorter equipment. These applications have not heretofore been open to the use of electrically resistive heat transfer techniques, instead requiring much slower and extremely noisy impact printing techniques.
In the so-called typical office applications, the criteria for setting minimum standards of clarity and quality are often largely subjective judgments left to the individual typing or printing the document and, accordingly, a high degree of variation exists. In the printing of documents to be sorted by magnetic reader/sorter equipment, however, the standards are extremely detailed, and critical image standards established by the American Banking Association for magnetic encoded images must be met.
Typical ribbons used today for impact printing of checks, negotiable documents, and the like, generally have an ink coating which is on the order of 65% or more magnetic oxide. Such a loading of magnetic oxide has been considered essential to obtain both visual print quality and the desired level of signal transmission for machine scanning. Yet such loadings are clearly impossible in thermal transfer applications, where the ink layer must melt and transfer to the paper or document substrate, because the melting points of the magnetic oxides are several orders of magnitude higher than the general limit at 150.degree. C. required to avoid melting the electrically resistive polymer substrate.
It is therefore, one object of the present invention to provide a ribbon for non-impact magnetic printing of checks and other documents traditionally handled and processed with the aid of magnetic reader/sorter equipment.
It is another object of the present invention to provide a magnetic ink composition useful in encoding checks and similar documents processed with the aid of magnetic reader/sorter apparatus.
It is yet another object of the present invention to provide a ribbon for non-impact erasure proof printing of checks and other negotiable documents.